1. Field of the Invention
The present invention relates to an optical pulse time spreading device for use in code division multiplexed optical telecommunictions, more particularly to an optical pulse time spreading device employing superstructured fiber Bragg gratings.
2. Description of the Related Art
With the spread of the Internet, the demand for telecommunications has grown rapidly, prompting the construction of high-speed, high-capacity optical fiber networks. To increase communication capacity, optical multiplexing is essential. Various optical multiplexing methods, including optical time division multiplexing (OTDM), wavelength division multiplexing (WDM), and optical code division multiplexing (OCDM) are currently undergoing intensive study.
Among these methods, OCDM provides operational flexibility in that there is no limit on the amount of time that may be allocated per bit. OCDM can also be combined with OTDM or WDM.
In OCDM, optical signals for a plurality of communication channels are encoded at the transmitting end, using the same wavelength or group of wavelengths but a different code for each channel. The encoded signals are combined into a single multiplexed signal. The multiplexed signal is decoded at the receiving end, using corresponding codes, to recover the original optical signal in each channel.
OCDM can dramatically increase the capacity of a WDM or OTDM system by enabling a plurality of communication channels to be carried on the same wavelength of light, or in the same time slot. OCDM also offers advantages in terms of security, since a transmitted signal cannot be decoded unless the receiver is in possession of the same code as the transmitter.
OCDM systems can be implemented in various ways. Phase coding, in which a pulse of coherent light representing a bit of data is converted to a train of chip pulses with various phase shifts, has received much attention.
A phase coding OCDM method using planar lightwave circuits (PLCs) including tapped delay lines and optical phase shifters for optical encoding and decoding has been described by Wada et al. in ‘A 10 Gb/s Optical Code Division Multiplexing Using 8-Chip Optical Bipolar Code and Coherent Detection’, IEEE Journal of Lightwave Technology, Vol. 17, No. 10, October 1999.
A phase coding OCDM method using transversal optical filters for encoding and decoding has been described by Sotobayashi in ‘Hikari fugo bunkatsu taju nettowaku” (Optical code division multiplexed networks), Oyo Butsurigaku, Vol. 71, No. 7 (2002), pp. 853-859).
A phase coding OCDM method using liquid-crystal light phase modulators for encoding and decoding has been described by Cong et al. in ‘An Error-Free 100 Gb/s Time Slotted SPECTS-CDMA Network Testbed’, Paper Th. 1.4.6, Vol. 3, ECOC 2005.
A phase coding OCDM method using arrayed waveguide gratings (AWGs) for encoding and decoding has been described by Cao et al. in ‘Spectral Encoding and Decoding of Monolithic InP OCDMA Encoder’, Paper We.3.6.6, Vol. 3, ECOC 2005.
A phase coding OCDM communication method using superstructured fiber Bragg gratings (SSFBGs) for encoding and decoding has been described by Nishiki et al. in ‘SSFBG wo mochiita OCDM yo iso fugoki no kaihatsu’ (Development of a phase coder for OCDM using an SFFBG), Technical Report of IEICE, OFT2002-66 (November 2002).
In all of these conventional methods, the different codes are different sequences constructed from the same limited set of phase shifts: four phase shifts (0, π/2, π, and 3π/2) in Sotobayashi's method, and just two phase shifts (0 and π) in the other methods.
An SSFBG and some of the other devices used in these conventional methods have the advantage of being passive optical components that do not consume electrical power. Passive optical components are also not limited by electrical signal processing speeds, so communication equipment using them can be easily adapted to handle future increases in transmission rates.
An SSFBG has the further advantage of being essentially just a length of optical fiber, with the same fiber geometry as the optical fiber used for transmission in optical communication systems employing the overlapped domain decomposition method (ODDM). An SSFBG can therefore be installed in an optical transmission system by fiber-to-fiber coupling, which is simpler than connecting an optical fiber to a different type of device such as a PLC.
A still further advantage of an SSFBG over a PLC or AWG is its small size and comparatively low optical loss.
When used for OCDM communication by the conventional method, however, an SSFBG provides an inadequate signal-to-noise ratio (auto-correlation to cross-correlation energy ratio). The basic reason is that the autocorrelation process used to decode the received signal includes much destructive interference. Further details will be given in the detailed description of the invention.
The same problem arises in the other conventional OCDM methods described in the references above.